A wide variety of types and configurations of instruments for sensing and measuring high frequency and microwave power are known. These prior art techniques suffer from several inherent drawback. These drawbacks include: mismatch errors, non-linearity, limitations on the frequency range of operation, and complexity of the required equipment.
The usually used dc bridge circuits set the dc resistance to a value which would make the input dc resistance and hence the RF input resistance equal to the characteristic resistance of the input transmission line. The change in direct current necessary to maintain the dc resistance constant in the presence of the ac power to be measured, provides a substituted dc power measurement for the ac power. This technique however is valid only if the RF and dc resistances are equal which normally they are not. The present invention also uses the substituted dc power measurement approach for measuring ac power but adjusts the dc input to compensate for changes in RF resistance as a function of ac frequency.
Methods are also known which utilize pairs of thermistors. Unless the two thermistors have exactly the same resistance and temperature coefficients of resistance which they normally do not, there will be a linearity error due to unequal division of the power.
When using a pair of thermistors, bypass capacitors are required because both thermistors should not be grounded. Capacitors are also used to prevent dc current from flowing through the RF input transmission line. These capacitors determine the frequency limits of operation.
Known methods also use bridges or feedback techniques to control the resistance of the thermistors and precise voltmeters to measure the dc voltage across the thermistors. In automated systems the voltmeter is also required to convert the dc voltages to digital signals for use by computer circuits.